Bradley, assuming they get their financial situation trending upward, will be the next team to leave the MVC. It may not be in the next 5 years, but within the next ten.
Two things about expansion that we've learned recently:
1) Football leagues are not going to add non-football schools. For the most part, football tends to be worth a lot more money when it comes to TV contracts and football schools don't want to share money with non FB schools. And yes, the MWC is a football conference.
2) The Big East will not add any public schools. It is a league driven by school presidents, as opposed to athletic directors, and they like the ability to make decisions in secrecy (being able to avoid freedom of information act requests and state boards of trustees that leak sensitive data like a sieve).
I expect the Big East to add schools, but it may not be for several years and will probably come when the BCS schools break away. Any private schools want to come aboard should start improving programs and facilities immediately. When BCS conferences break away and start their own basketball tourney, the Big East will be included.
I also think the day will come when some MVC schools are going to have to decide if it is worthwhile to field football programs.